It is possible to detect radiation, such as infrared (IR) radiation, using one of two general types of sensors. A first sensor type (photon sensor) detects the presence of the radiation by detecting the direct interaction of the radiation with the atomic lattice of the sensing material in the sensor, and a second sensor type (thermal sensor) detects the radiation by detecting a temperature change in the sensor due to the absorbed radiation. The temperature change causes a physical parameter of the sensor, such as resistance or voltage, to change and therefore can be detected by electronic circuitry.
With thermal sensors, thermal mass (MTH) and thermal conductivity (CTH) determine the response time of the system, where the response time is proportional to MTH/CTH. Generally, it is desired to have the greatest temperature change per unit of radiant power on the sensor, implying a sensor with a small thermal mass and/or a well thermally isolated sensor. However, if the sensor is too well isolated, the heat will not dissipate fast enough, therefore extending the response time of the thermal sensor.
With reference now to FIGS. 1a and 1b, there are shown diagrams illustrating prior art implementations of thermal sensor pixels. The diagrams shown in FIGS. 1a and 1b illustrate two different prior art implementations of a single pixel in a thermal sensor array. The diagram shown in FIG. 1a illustrates a single pixel 100 used in the thermal sensor array. The pixel 100 includes a detector (or absorber) 105 that includes a pair of legs 110 that elevates the detector 105 from the remainder of the thermal sensor array to thermally isolate the detector 105 from other detectors in the thermal sensor array. Conductors 115 and 120 couple the detector 105 to electronic circuitry and permit the detection of changes in physical parameters, such as resistance and voltage.
The diagram shown in FIG. 1b illustrates a single pixel 150 used in a thermal sensor array. The pixel 150 includes a detector 155 that is coupled to a rim 160 via serpentines 165. The use of the serpentines 165 yields a high coefficient of thermal resistivity due to the small cross-section of the serpentines 165, which provides electrical connectivity while minimizing thermal transfer. The pixel 150 can be created using a bulk etching process, allowing for mass production of thermal sensor arrays.
One disadvantage of the prior art is that the design of the pixels, although yielding good thermal performance, has a poor fill factor for the thermal sensor array due to the underlying support structure of the pixels, such as the legs 110, the rim 160, and the serpentines 165, consuming a significant amount of integrated circuit real estate. A low fill factor results in a thermal sensor array that generally cannot be readily scaled since a thermal sensor array with a large number of pixels may be unnecessarily large.
Another disadvantage of the prior art is the design of pixels with significant thermal mass, which can increase the response time of the pixels and the thermal sensor array. This can lead to a reduction in the overall performance and responsiveness of the thermal sensor array.